Defects in the form of structural flaws, process residues, and external contamination occur during the production of objects such as Printed Circuit Boards, semiconductor wafers, artwork or reticles. Defects are typically detected by a class of instruments called defect scanners (or automatic optical inspection systems). Such instruments automatically scan the object's surfaces and detect morphological or design anomalies using a variety of techniques. The location of these anomalies with respect to the patterns on the object's surface is recorded. This information, or “defect map,” is stored in a computer file and sent to a defect review station.
Using the defect map to locate each defect, a human operator observes each defect under a microscope and characterizes each defect according to type (e.g., particle, pit, scratch, or contaminant). Information gained from this process is used to correct the source of defects, and thereby improve the efficiency and yield of the semiconductor production process. Unfortunately, people are relatively slow and are quickly fatigued by the highly-repetitive task of observing and characterizing defects.
Methods of automatically characterizing defects, collectively known as Automatic Defect Characterization (ADC) or Automatic Optical Inspection (AOI) have been developed to overcome the disadvantages of manual defect characterization.
One of the well-known algorithms that is used by AOI systems is a skeleton comparison algorithm. This algorithm skeletonizes the objects of both images—the reference image and the inspected image—and compares the skeletons' junctions and the skeletons' ends in both of the two skeleton images. Skeletonization must be done in real time and in the inspection station. Moreover, the skeletonization process requires a lot of computing source. Consequently, the main disadvantage of these inspection systems is that they are complicated and expensive.
It would be advantageous to have a method and a system, which once skeletonize objects of a reference image and compare this skeletonized image with a real image of the inspected object. This method reduces significantly the costs of the inspection scanners that intend to supply only a real image without real time sketetonization. Moreover, another computer wherein the processing time is not very critical can do the process.